An arm type valve operating device using a rocker arm opening and closing an intake valve or an exhaust valve through rotation of a cam is of two types: an end pivot type in which a cam shaft is arranged above a swingably-supported rocker arm, and a center pivot type in which a cam shaft is arranged below a rocker arm.
In the end pivot type, the rocker arm is pushed down by a cam provided on the cam shaft to open the valve; and, in the center pivot type, one end portion of the rocker arm is pushed up by a cam provided on the cam shaft to open the valve with the other end portion of the rocker arm.
That is, in the center pivot type, the rocker arm is pivoted to the rocker shaft, and is swingable around the rocker shaft axis. Thus, the rocker arm and the rocker shaft are in sliding contact with each other. Thus, it is desirable that the contact surfaces be not worn.
In a related-art technology, to achieve an improvement in terms of durability, lubricant oil is supplied to the sliding portions between the rocker arm and the rocker shaft (Patent Document 1).
In the case of the end pivot type, a valve cap coming into contact with the rocker arm may be arranged at the forward end of the valve stem portion of the valve. It is necessary for such a valve cap to be superior in durability. Thus, in a related-art technology, an attempt is made to achieve an improvement in durability (Patent Document 2).
In the valve cap disclosed in Patent Document 2, the abutment portion of the valve cap in press contact with the rocker arm is shifted circumferentially little by little each time the rocker arm comes into press contact therewith; further, there is used a high speed tool steel material.
An adjust screw is disposed adjacent to the rocker arm; in the center pivot type, the adjust screw is held in sliding contact with a valve; in the end pivot type, it is held in sliding contact with a pivot receiving member. Thus, it is desirable for the contact surfaces not to be worn.
In a related-art technology, in order to achieve an improvement in durability, a lubricant member is provided in an engagement portion between a screw and a socket receiving the same (Patent Document 3).
According to the disclosure of Patent Document 3, engine oil, for example, is used for the lubricant member, and the engagement portion of the screw and the socket is filled with oil. Patent Document 3 further discloses use of alloy steel that has undergone surface-hardening treatment for one of the screw and the socket.
In the case of the end pivot type, the valve receiving portion of the rocker arm and the valve are in sliding contact with each other, so the valve receiving portion must be superior in durability. In view of this, in a related-art technology, an attempt is made to achieve an improvement in durability (Patent Document 4).
In the rocker arm disclosed in Patent Document 4, a diamond-like carbon film is formed on the portion thereof coming into contact with the valve. The surface roughness Ry (maximum height in μm) of the diamond-like carbon film is 3.2 or less, and the thickness of the diamond-like carbon film ranges from 1.0 to 5.0 μm, with the adhesion property being 40 N or more.    Patent Document 1: JP 07-150921 A    Patent Document 2: JP 2001-152817 A    Patent Document 3: JP 08-35408 A    Patent Document 4: JP 2002-97906 A